I have a few friends that have 6.7L cummins crew cab 4x4's and the mileage sucks on them too. If you want to get some mileage you have to get that DPF off from your truck....check out spartentechdiesel. Matt has a tune thats making 500+RWHP from the 6.4L with just tuning and removing the DPF/DOC. Not to mention most guys removing the DPF's are getting in the 17+mpg range. Same goes for the 6.7L its a turd for fuel mileage as well. But I have heard several people getting much better mileage once they remove the dpf and block EGR coolers.

the 6.7s are a great damn motor my buddy gets 22mpg with his on the high way and 15 in town. and for those who are gonna ask, well whats all done to it: 6 in rough country lift, 37x12.5x17's cooper stt's, chip, strait piped 4 in exhaust with dpf delete, sb cold air, south bend dual disk clutch, and i cant remember what else right now. and it tows like there is nothing behind it